We have established cDNA clone libraries from human fetal organs and from HeLa cells. These libraries, in addition to future cDNA clone collections we propose to construct, will be used for the isolation of organ- and chromosome 21- specific genic sequences. A major long-term objective is elucidation of the pathogenesis of Down syndrome at the molecular level. We have shown previously that gene dosage for chromosome 21-specific transcripts can be detected using nucleic acid hybridization. We propose to extend this work by cloning genic sequences on human chromosome 21 expressed during development in the fetal organs relevant to the major clinical disabilities of Down syndrome. Isolation of chromosome 21-specific sequences will be accomplished by cross-hybridizations between (1) a recombinant library of phage containing genomic DNA from a mouse-human somatic cell hybrid having a single human chromosome, number 21 and (2) pools of non-repetitive cDNA clones we have derived from the above cDNA libraries. The cloned chromosome 21-specific sequences will be mapped to sub-regions of chromosome 21, used in gene dosage experiments, and examined to see if they code for membrane-associated proteins which might play a key role in mediating the developmental disabilities of Down syndrome. We also propose to isolate specific genic sequences expressed in abundance in various fetal organs. Specifically, we will attempt to isolate myoglobin from the fetal heart cDNA collection, and apolipoproteins A-I and E, and glucose 6-phosphate dehydrogenase from the fetal liver cDNA collection. The proposed screen for G6PD, which will utilize a kindred deficient in G6PD cross-reacting material, is designed as a prototype experiment for the usage of genetic substrates from human mutants to probe for specific genic sequences in clone libraries.